Resistance material and method of making same



Patented July 2, 1940 UNITED STATES PATENT. OFFlC E Bertil Stalhanc, Stockholm, Sweden No Drawing. Application September 13, 1931, Serial No. 163,697. In Sweden October 17,

The present invention relates to semi-conducting materials the electrical resistance ofwhieh falls automatically more or less rapidly atan .increased voltage drop across the materialso that the current through the latter increases more rapidly than the voltage impressed. thereon and also to process of making such materials. Such resistances have lately been extensively used, inter alia, as excess-voltage protections or 19 lightning arresters in electrical plants, a number of resistance blocks, sensitive to the voltage, being generally connected in series with spark gaps and fuses or mechanical interrupting devices.

- Also in various other regulating and switching devices such voltage-sensitive resistanoes are of importance.

It has been previously proposed to make re.- sistance blocks of this type of granular semiconductors, such as silicon carbide or carbon, for example graphite, which materials are held together by a bindingsubstance.

In the manufacture of the bio ks the granular resistance material with the ad ition of bindin material is pressed in a suitable mould and is then allowed to harden which may be caused or facilitated through drying and burning. It has been proved in practice .that it is very difllcult to obtain a binding material which gives satisfactory mechanical as well as electrical properties 30 to the resistance block. On the one hand, the

resistance mass must have a sufficient mechanical resistivity and, on the other, the binding material must not insulate the semi-conducting particles from one another after being dried or burned. a Extensive tests have shown that the semi-conductin particles do not-per se possess a resistance which varies with the voltage whereas the 1 sensitivity to the voltage actually obtained has its origin in the contact points between abutting seir i conductor particles or in the microspaces- 50 bide have particularly advantageous properties in regard to the voltage-sensitivity of the contact resistance between adjacent crystals in a set or mass of crystals. In order to'utilize to the full extent the advantageous properties of silicon 55 carbide in this respect very particular require- 500 degrees centigrade.

8 Claims. (cram-7c) gments must be imposed on the material, intended to bind together the crystal mass.

In. many instances, mixtures comprising gypsum, hydraulic cement, or cement of the Sorel type have been used as binding materials in 5 which case the product is likely to be sensitive through their sintering at high temperatures in which case the result obtained is highly dependent upon the burning process and the temperature. In binding materials of this type the glass formation and the crystallization during the sintering have a great influence upon the electrical properties of the product. \At higher burning temperatures than 1100 degrees centrigrade 'there is further a tendency to a superficial oxidation of the granular material for which reason the process is quite out of control of the op- W erator in many instances. In any case the percentage of rejected articles is considerable in the manufacture.

The present invention has for its object to eliminate said inconveniences and consists substantially therein that the granul icon carbide is mixed with binding material in luding, on the one hand, one or more of the foil wing acidcontaining components, i. e., phosphoric acid or an anhydride thereof, acid phosphates of alka 80 line earth metal, ammonium phosphates or analogus compounds of arsenic acid and, on the other, one or more components including oxides or hydroxides of aluminum, iron, chromium or zirconium. The mass obtained is then pressed 36 in a suitable mould and dried and burned.

If all the constituents are solid they may first be disintegrated, unless already in granularor pulverized state respectively, and are then mixed and, if found suitable, water is added thereto. 40

If one or more of the components, such as orthophosphoric acid, is or are liquid or soluble in water, the solid constituents and those which are insoluble in water may first be mixed in 'dry condition and then the binding material, being itself" a liquid'or dissolved in a liquid, is added thereto. Q

. In the present method the fixation of the binding material takes place through the formation of phosphates or arsenates of aluminium, iron,

chromium, or zirconium which reaction starts already at a low temperature, generally less than During a continued heating the condition of the binding material is modified only through the driving ofi of water or hydration, if any, and through the crystallize- .tion of amorphous constituents, etc. Extensive tests have shown that the entire process is under complete control and that a fully uniform prodnot is obtained. The product is highly insensitive to humidity and shows no aging phenomena.

' sible with hitherto known following substances, i.

2 as bauxite or laterite AlzOa.aq,'having a varying content of iron and When resistance materials are manufactured "according to the present invention using silicon carbide as base material it is possible to obtain a higher voltage-sensitivity than has been posbinding materials. Resistance blocks having a diameter of 7.5 centimeters and a thickness of 2 centimeters may easily be obtained in which the degree of voltage sensitivity rises with the amperage according to the table following:

Voltage bound water), chrpmites, chromates or chromium oxide (Ci-203) or zirconium oxide (ZrOz) or zircite (i e., ore of zirconium oxide, having a varying content of colloidal zirconium oxide, iron oxide, etc.).

Mixtures of the "above mentioned substances can be used. The leading principle is that such combinations are selected that. phosphates or arsenates .of said metals are formed during the heating. The acid alkaline earth metal phosphates and ammonium phosphates or the analo-' gous arsenic compounds are decomposed in the reaction while giving off water or ammonia re-.

spectively, phosphoric acid or arsenic acid being released and reacting with the other components.

It has been proved that generally 0.5 to 5 parts by weight of combination with 2 to 20 parts of oxides or hydroxides respectively are suflicient to bind 100 parts granular semi-conducting material.

Good nesults have been obtained for example by adding to 100 parts by weight of granular silicon carbide 5 to 10 .parts by weight of aluminum hydroxide (such as bauxite or laterite) or chromium oxide (chrome-green) or deposited zirconium oxide together with 5 to '10 parts. of

weight by a solution, prepared of 1 part by volume of phosphoric acid of specific gravity 11.70 and 2 partsby volume of water, and the pressing, drying, and heating'ofsthe. mass at temperatures from 500 up to 1100 degrees centigrade.

In" combining laterite, i .e., a bauxite having a large content, such as 25 to 35 per cent, of bound water, with phosphoric acid the advantage is gained that the formation of aluminum phosphate commences at a temperature of about Amperage Ohms monocalcium ortho-phosphat e the acid-carrying constituents in' degrees centlgrade and that the pressed articles therefore obtain a good mechanical resistivity at this lowtemperature. By heating to higher temperatures, the electrical properties of the material may be modified.

What is' claimed is:

1. In the manufacture of semi-conducting materials having a granular structure and a voltagesensitive resistance, the process which comprises mixing granular silicon carbide with at least two chemical compounds capable of reacting during the subsequent burning step to form a compound selected from the class consisting of the phosphates and arsenates of aluminum, iron, chromium and zirconium, then molding the resulting mixture into block form and burning to bind the particles of the mixture to form a semi-conducting material.

2. In the manufacture of semi-conducting materials having a granular structure and a voltagesensitive resistance, the process which comprises mixing granular silicon carbide with an acidcontaining compound, selected from a first class consisting of the phosphoric acids and their anhydrides, alkaline earth metal acid-phosphates, ammonium phosphate and the corresponding compounds of arsenic, and also with a metal compound selected from a second class consisting of aluminum, iron, chromium and zirconium compounds, the compounds of said second class being capable of reacting with the compounds of said first class during the following burning step with the formation of the corresponding phosphates and arsenates, then molding the resulting mixture into block form and burning to form a semiconducting material.

3. In the manufacture of semi-conducting materials having a granular structure and a'voltagesensitive resistance, the process which comprises mixing granular silicon carbide with metal coma pounds selected from a class consisting of the oxides andihydroxides of aluminum, iron, chromium and zirconium, and also with acid-containing compounds capable of reacting with said metal compounds during the subsequent burning step with the formation of the corresponding metal phosphates and arsenates, then molding the resulting mixtures into block form and buming to form semi-conducting materials.

4. In the manufacture of semi-conducting ma- 5 terials having a granular structure and a voltagesensitive resistance, the process which comprises mixing granular silicon carbide with an acidcontaining compound, selected from a first class consisting of the phosphoric acids and their anhydrides, alkaline earth metal acid-phosphates, ammonium phosphate and the corresponding compounds of arsenic, and also with a metal compound selected' from a second class consisting of the oxides and hydroxides of aluminum, iron, chromium and zirconium, the compounds of said first class being 'capable' of reacting'with the compounds of said second class during the folfhwing burning step with the formation of the corresponding 'metal phosphates and arsenates, then molding the resulting mixture into block form and burning to form a semi-conducting material. 5. The process of claim 2 wherein about 100 parts by weight of silicon carbide are mixed with about 0.5 to 5 parts by weight of said acid-containing compound and-with from aboutv 2 to 20 parts by weight of said metalgcompound.

6. In the manufacture of semi-conducting materials having a granular structure anda voltagesensitive resistance, the process which comprises mixing granular silicon carbide with phosphoric acid, finely divided hydrate of aluminum oxide and finely divided chromium oxide in such proportions as to react during the subsequent burning step with the formation of aluminum and chromium phosphates, molding the resulting mixture and burning to bind the particles of the mass forming a semi-conducting material.

'7. An electrical resistance unit, suitable for use in lightning arresters, over-voltage protective devices and the like, comprising, as an aggregate, a mass of silicon carbide crystals bound together substantially in block form by a sintered matrix comprising at least one compound formed in situ, selected from a class consisting of the phosphates and arsenates of aluminum, iron, chromium and zirconium, in such manner that said mass of crystals is electrically conducting, forming a voltage-sensitive resistance, highly insensitive to humidity and showing substantially no aging phenomena. Y

- 8. The electrical resistance unit of claim 7 wherein said matrix comprises the phosphates of 1 aluminum and chromium.

BERTIL' STALHANE. 

